This paper describes a method to measure the 3D-microstructure of a material which can be used to perform fracture simulations. A model concrete is made and the 3D structure is obtained with a CTscanner. Uni-axial tensile tests are performed on cylindrical specimens of the model concrete a regular concrete and of a mortar. The model concrete shows more micro-cracking, a more tortuous crack path, a lower tensile load and a less brittle behaviour compared to the mortar and the regular concrete. Furthermore it is found that the opening of the crack is more uniform when the material is more heterogeneous, which results in a more stable fracture.